CH6 - CPU Scheduling

CPU-I/O Burst Cycle

Process execution consists of a cycle of CPU execution and I/O wait

_______ distribution is of the main concern

CPU Burst

___________ selects from among the processes in ready queue, and allocates the CPU to one of them

Short-term scheduler

CPU scheduling decision when a process

1. Switches from running to waiting state

2. Switches from running to ready state

3. Switches from waiting to ready

4. Terminates

Scheduling under 1 and 4 is ____________

nonpreemptive

Dispatcher

gives control of the CPU to the process selected by the short-term scheduler

Dispatcher includes _________

• switching context

• switching to user mode

• jumping to the proper location in the user program to restart that program

Dispatch latency

time it takes for the dispatcher to stop one process and start another running

Scheduling Critieria

1. CPU utilization

2. Throughput

3. Turnaround time

4. Waiting time

5. Response time

CPU utilization

keep the CPU as busy as possible

Throughput

# of processes that complete their execution per time unit

Turnaround time

amount of time to execute a particular process

turnaround time=completion time-arrival time

Waiting time

amount of time a process has been waiting in the ready queue

waiting time=turnraround time-burst time

Response time

amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment)

Convoy effect  

Short process first and then long process

Starvation

low priority processes may never execute

Aging

as time progresses increase the priority of the process